Generally, to produce a uniform trace when plotting a line segment of printing an alphanumeric character, prior art thermal printers and plotters often included circuitry which provided uniformity of trace as a function of such factors as battery voltage level or slew rate of the print-plot head. Thermal printer-plotters of this type are described, for example, in U.S. Pat. No. 3,934,695, entitled "Method and Apparatus for Enhancing and Maintaining Character Quality in Thermal Printers," issued Jan. 27, 1976 to Albert W. Kovalick, and in U.S. Pat. No. 3,986,011 entitled "Printer-Plotter System," issued Oct. 12, 1976 to John S. Poole, et al.
In thermal printers where the print rate is on the order of 10 characters per second or less (e.g., 5 milliseconds power on pulse with a minimum period of 35 milliseconds), the above techniques for producing uniform characters are generally sufficient. However, when the same thermal printers are used at a faster printing rate (i.e., shorter period), the individual thermal elements in the character matrix produce printed images of varying density resulting in character images that are hard to read.
The underlying cause of these problems is that the same amount of power is typically applied to each thermal element each time it is actuated independent of the residual temperature of the print head at the time of actuation. Thus, a previously energized thermal element (e.g., an element energized to print a previous line segment or alphanumeric character) may cool to a temperature level that is higher than the ambient temperature level of an element not previously energized, or to ambient temperature if it has had sufficient time to fully cool. Hence, the previously energized thermal element, that has not cooled completely will heat to a higher temperature than the not previously energized thermal element, when it is activated, resulting in the printing of a higher density image. If the thermal elements are continually reactivated at the faster print rate, the printing temperature will become increasingly greater until it reaches a new steady-state with the inherent danger that the thermal element may become overheated causing its failure.
To overcome these problems, a thermal element drive circuit is needed wherein the amount of power applied to the thermal element is proportional to the amount of power necessary to reheat the thermal element to substantially the desired printing temperature. The instant invention provides such a solution.